Stephanie L. Outcalt

A Modified Benedict–Webb–Rubin Equation of State for the Thermodynamic Properties of R152a (1,1‐difluoroethane)

A modified Benedict–Webb–Rubin (MBWR) equation of state has been developed for R152a (1,1‐difluoroethane). The correlation is based on a selection of available experimental thermodynamic property data. Single‐phase pressure–volume–temperature (PVT), heat capacity, and sound speed data, as well as second virial coefficient, vapor pressure, and saturated liquid and saturated vapor density data, were used with multi‐property linear least‐squares fitting to determine the 32 adjustable coefficients of the MBWR equation. Ancillary equations representing the vapor pressure, saturated liquid and saturated vapor densities, and the ideal gas heat capacity were determined. Coefficients for the equation of state and the ancillary equations are given. Experimental data used in this work covered temperatures from 162 K to 453 K and pressures to 35 MPa. The MBWR equation established in this work may be used to predict thermodynamic properties of R152a from the triple‐point temperature of 154.56 K to 500 K and for pressu...

Equations of state for the thermodynamic properties of R32 (difluoromethane) and R125 (pentafluoroethane)

Thermodynamic properties of difluoromethane (R32) and pentafluoroethane (R125) are expressed in terms of 32-term modified Benedict-Webb-Rubin (MBWR) equations of state. For each refrigerant, coefficients are reported for the MBWR equation and for ancillary equations used to fit the ideal-gas heat capacity and the coexisting densities and pressure along the saturation boundary. The MBWR coefficients were determined with a multiproperty fit that used the following types of experimental data: PVT: isochoric, isobaric, and saturated-liquid heal capacities; second virial coefficients; and properties at coexistence. The respective equations of stale accurately represent experimental data from 160 to 393 K and pressures to 35 MPa for R32 and from 174 to 448 K and pressures to 68 MPa for R125 with the exception of the critical regions. Both equations give reasonable results upon extrapolation to 500 K and 60 MPa. Comparisons between predicted and experimental values are presented.

A theoretically-based calibration and evaluation procedure for vibrating-tube densimeters

Abstract A calibration procedure for vibrating-tube densimeters is developed which properly accounts for the effects of pressure and temperature on the Youngs modulus and internal volume of the vibrating-tube. The calibration equation is based on the theoretical dependence of the Youngs modulus, the compressibility, and the thermal expansion coefficient of the tube material on temperature and pressure. Experience shows that the vibration period of the evacuated tube can shift a small amount over time as the stresses in the tube and welds age. Therefore, the calibration equation is formulated relative to a vacuum reference period to adjust for these shifts. A first-order approximation of our theoretically-based equation is also derived. The calibration procedure is accomplished in two parts. First, the evacuated tube is calibrated to characterize the elastic modulus and linear thermal expansion coefficient of the tube as a function of temperature. Second, the change of the internal volume of the tube with temperature and pressure is characterized using two or more well-characterized calibration fluids. A procedure for choosing calibration fluids, temperatures, and pressures for the calibration points is developed. The densimeters are thoroughly tested with a variety of gases and liquids to show the validity of the equation over the calibration range. Finally, the magnitude of the temperature and pressure corrections are shown using propane+i–butane as a test system.

Coexisting densities and vapor pressures of refrigerants R-22, R-134a, and R-124 at 300–395 K

Abstract The results of the investigation are presented in two parts. Part I, given in this paper, presents the experimentally measured coexisting densities and vapor pressures for the refrigerants R-22 (chlorodifluoromethane), R-134a (1,1,1,2-tetrafluoroethane), and R-124 (1-chloro-1,2,2,2-tetrafluoroethane) from 300 K to near their respective critical points. In addition, compressed liquid and supercritical densities were measured for R-22 and compared to literature values. The compressed R-22 densities agreed within experimental error with those of Kohlen et al. (Kohlen R., Kratzke, H. and Muller, S., 1985. Thermodynamic properties of saturated and compressed liquid difluorochloromethane. J. Chem. Thermodyn., 17: 1141-1151). Considerable discrepancies were found in the literature for R-134a and R-124 coexisting densities and vapor pressures. For both R-134a and the R-124, at least one set of data from the literature agreed with our results. The analysis of the measurements to determine critical densities which are internally consistent with our experimental measurements is presented as Part II (Van Poolen, L.J., Niesen, V.G., Holcomb, C.D. and Outcalt, S.L., 1994. Critical densities from coexisting density data: application to refrigerants R-22, R-134a, and R-124. Fluid Phase Equilibria, in press) in a separate paper.

Coexisting densities, vapor pressures and critical densities of refrigerants R-32 and R-152a, at 300-385 K

Abstract Holcomb, C.D., Niesen, V.G., Van Poolen, L.J. and Outcalt, S.L., 1993. Coexisting densities, vapor pressures and critical densities of refrigerants R-32 and R-152a at 300-385 K. Fluid Phase Equilibria , 91: 145-157. Experimental measurements for the vapor pressures and coexisting densities are presented for the refrigerants R-32 (difluoromethane) and R-152a (1,1-difluoroethane) from 300 K to near thier respective critical points. In addition, the coexisting density measurements have been analyzed to determine an internally consistent critical density using the critical liquid volume fraction method. Experimental results have been correlated and are in good agreement with existing literature values for each compound.

Thermophysical Properties Measurements of Rocket Propellants RP-1 and RP-2

The density, speed of sound, and viscosity of two rocket propellants (RP-1 and RP-2) have been measured. Densities were measured with two different instruments. Data at ambient atmospheric pressure were obtained with a rapid characterization instrument from 278.15 to 343.15 K that measured the speed of sound and density of the liquids in parallel. Adiabatic compressibilities derived from that data are included here. Densities of the compressed liquids were measured in an automated apparatus from 270 to 470 K and pressures to 40 MPa. Viscosities of the two liquids were measured in an open gravitational capillary viscometer at ambient atmospheric pressure from 293.15 to 373.15 K. The measurement results are consistent with compositional differences between the two samples. Correlations have been developed to represent the measured properties within the estimated uncertainties of the experimental data and to allow physically meaningful extrapolations beyond the range of the measurements.

An Equation of State for the Thermodynamic Properties of R143a (1,1,1-Trifluoroethane)

Thermodynamic properties of 1,1,1-trifluoroethane (R143a) are expresed in terms of a 32-term modified Benedict-Webb-Rubin (MBWR) equation of state. Coefficients are reported for the MBWR equation and for ancillary equations used to lit the ideal-gas heat capacity, and the coexisting densities and pressure along the saturation boundary. The MBWR coefficients were determined from a multiproperty fit that used the following types of experimental data:PVT: isochoric, isobaric, and saturated-liquid heat capacities: second virial coefficients: speed of sound and properties at coexistence. The equation of state was optimized to the experimental data from 162 to 346 K and pressures to 35 MPa with the exception of the critical region. Upon extrapolation to 500 K and 60 MPa, the equation gives thermodynamically reasonable results. Comparisons between calculated and experimental values are presented.

A Small-Volume Apparatus for the Measurement of Phase Equilibria

An apparatus has been designed and constructed for the measurement of vapor-liquid equilibrium properties. The main components of the apparatus consist of an equilibrium cell and a vapor circulation pump. The cell and all of the system valves are housed inside a temperature controlled, insulated aluminum block. The temperature range of the apparatus is 260 K to 380 K to pressures of 6 MPa. The uncertainty of the temperature measurement is 0.03 K, and the uncertainty in the pressure measurement is 9.8 × 10−4 MPa. An automated data acquisition system is used to measure temperature and pressure at equilibrium. The apparatus has been performance tested by measuring the vapor pressures of propane, butane, and a standard mixture of propane + butane.

Near-saturation (P,ρ,T) and vapor-pressure measurements of NH3, and liquid-phase isothermal (P,ρ,T) and bubble-point-pressure measurements of NH3+H2O mixtures

Abstract Near-saturation pressure, density, and temperature ( P , ρ , T ) and vapor-pressure measurements for NH 3 are reported over a temperature range from 279 to 392 K. Liquid-phase isothermal ( P , ρ , T ) and bubble-point-pressure measurements for two standard mixtures of NH 3 +H 2 O ( x NH 3 =0.8360 and 0.9057 mole fraction) are reported over a temperature range from 280 to 379 K and at pressures to 7.7 MPa. These data are compared to literature data and correlations and agree within ±3% for bubble-point pressures, ±0.005 g/cm 3 for liquid densities, and ±0.0011 g/cm 3 for vapor densities. A consistent data set for equation-of-state optimization at high concentrations of NH 3 is proposed.

Critical densities from coexisting density data: application to refrigerants R22, R134a, and R124

Abstract Based on a mass balance and the behavior of the critical liquid volume fraction, appropriate forms of the rectilinear diameter for coexisting densities are dervied. For the refrigerants R22, R134a, and R124, a single temperature-dependent term in the rectilinear diameter was necessary to fit experimental vapor and liquid coexisting densities to determine the critical density. Internal consistency tests of coexistence density data and critical density values are developed. Critical density values for R22, R134a, and R124 are in good agreement with published values and with values calculated from published coexistence density data. Critical density values found in this study are 523.65 &+- 1.07 kg m 3 for R22, 513.02 &+- 1.98 kg m 3 for R134a, and 559.76 &+- 1.54 kg m 3 for R124.